Much work has been done in the field of liquid level detectors. The prior art work has ranged from floatation sensors to capacitance detectors wherein the capacitance changes as a result of a fluid level change. For example, see U.S. Pat. No. 5,287,086--Gibb, wherein the rise in level of the conductive liquid causes an oscillator to switch its mode of oscillation. Others have used a pair of plates between which a non-conducting liquid level is detected by comparing it to a reference value, for example, see U.S. Pat. No. 4,806,847--Atherton, et al. A number of prior art attempts have utilized RF frequencies in liquid level detection systems, for example see U.S. Pat. No. 4,800,755--Fathauer, et al., which utilizes a phase detector for detecting variations in phase angle as a function of probe capacitance.
However, none of the prior art devices provide the advantages of Applicant's invention, including the use of audio frequency for repeatability and reliability of results, sensors that can be mass produced relatively inexpensively and which will work in a conductive liquid where the conductive liquid is in a grounded container or otherwise has circuit ground potential, and which provides a circuit which may be safely utilized in connection with combustible liquids such as gasoline and diesel fuel.